Work from this and other laboratories established that an internal pool of Ca2 ion is mobilized in the response of liver cells to a variety of hormonal stimuli. We have now obtained evidence that this hormone-responsive pool of Ca2 ion is localized in the mitochondria of the hepatocyte. This conclusion is based upon studies of intact cells utilizing the fluorescent chelate probe, chlortetracycline, and of digitonin-treated cells whose Ca2 ion content was determined by the metallochromic indicator dye, arsenazo III, and upon experiments examining the Ca content of isolated mitochondria by atomic absorption spectroscopy. Some of these results have been confirmed and the conclusions supported by recent work from other laboratories. Continuation of these studies will examine the properties of Ca2 ion uptake by isolated liver microsomes. In particular, we will determine the effects of phosphorylation in vitro upon subsequent accumulation of Ca2 ion. We will also explore application of electron microanalysis for examination of intracellular distribution of Ca in hepatocytes and sections of liver tissues and examine a new technique for determination of intracellular pH.